non-aspirated voiceless stops versus their voiced counterparts before a vowel

Question

Is there a real distinction in say, a spectrogram, between unaspirated voiceless stops and their voiced counterparts before a (voiced) vowel? For example, /ka/ and /ga/. Are they actually different phonetically? And if so, how different.... picoseconds of voicelessness? Or is it significant?

My question comes from noticing that in Zulu, an aspirated /k/ often becomes non-aspirated when there's a nasal preceding it I believe (e.g., inkukhu (chicken) instead of inkhukhu), and it sounds very close to a /g/.... particularly when followed by a vowel. So, my hypothesis is that the two sounds become so similar, that they probably lose their ability to be distinct in that environment. Anyone know any specifics?

Answer 1

Phonetically, the main theory I've heard is that voiced/voiceless/aspirated consonants are distinguished by voice onset time. VOT is the time delta between when the consonant stops and when the vocal folds start vibrating.

If the VOT is positive, then there's a gap between the consonant ending and the vowel beginning. This is aspiration.

If the VOT is close to zero, then the consonant ends and the vowel begins right after. This is a "tenuis" (= "normal") consonant.

If the VOT is negative, then the vocal folds are vibrating during the consonant itself. This is a voiced consonant.

In other words, aspirated — tenuis — voiced are all different points on a continuum. Some languages have all three as distinct, like Ancient Greek, while others have only two. And the ones with two categories can choose different places to draw the line: French has strongly voiced [d] and barely unvoiced [t], while English has strongly aspirated [t] and barely voiced (or even not voiced at all!) [d].

It's entirely possible for an aspirated consonant to become unaspirated or voiced after a nasal—nasals are generally voiced, so this is straightforward assimilation. However, it's also possible for the opposite to happen! In Swahili, for example, tenuis stops became aspirated after nasals.

Answer 2

In Zulu is there is a contrast between aspirated voiceless, unaspirated (weak) ejective, and ostensively voiced stops. Aspirated consonants deaspirate after a nasal, so /izim-phaphe/ → [izim-paphe] 'feathers', cf. /izim-pete/ → [izim-pete] 'knock-kneed person'; see also your example "chicken". While /k, kh/ neutralize, /g/ remains distinct. However, distinguishing k and g (especially) but also t vs d is difficult. The voiced obstruents cause breathy voicing on the following vowel, and in fact they may be entirely devoiced during the closure if not preceded by a nasal. The main phonetic correlate of voicing of obstruents in Zulu (Xhosa, Swati) is causing breathiness on the vowel. There are a few contexts in Swati and Zulu where breathiness is not conditioned by a consonant (e.g. the first syllable of yebo "yes").

Long voice onset time is sufficient to identify aspirated stops. Breathy voicing and related pitch-lowering should be enough to distinguish b,d,g from unaspirated p,t,k, in case ejection is weak for a speaker / token.

Answer 3

The main difference between [k] and [g] is situated at the level of vocal folds. When [g] is pronounced the vocal folds vibrate, but not in the case of [k]. This situation is easily distinguishable on a spectrogram. [g] has what it is called a voice bar which does not appear on the spectrographic representation of [k]. Here a course which evokes the voice bar: http://ec-concord.ied.edu.hk/phonetics_and_phonology/wordpress/learning_website/chapter_3_consonants_new.htm